New research is showing that it is possible to make new, nutrient rich, productive soil on your farm without waiting for eons, simply by changing your crop rotation and management.
The research, led by Dr. Stuart Grandy at the University of New Hampshire, is revising our conventional understanding of how soil organic matter (SOM) is formed, suggesting that the process is a lot quicker and easier than we thought.
“We used to think that SOM was made up of the leftover bits from decomposition left behind by microbes, so whatever plant materials microbes did not break down fully became SOM,” says Grandy. “That leads to the idea that you want to add organic matter (plant residues) to soils that is very hard for microbes to break down if you want to build SOM. But in recent years we have realized that most SOM is made up of microbial residues. That creates a very different perspective on how SOM is formed and how to manage it. What we are looking at now is cultivating microbial communities that have characteristics that contribute to building SOM.”
“It’s a new understanding of how soil is built,” says Michael Thiele, Manitoba Grazing Club Coordinator. “It means our ability to build soil is much faster than what we had traditionally thought it to be. We used to think that soil is built on a glacial scale, so a very long timeframe, but if microbes form the basis of soil formation, as they live and die, expand and contract, we can build soil in a much shorter period of time.”
“What becomes really important in determining how much soil organic matter (SOM) we are going to build in a system, is not the amount of lignin going into the soil, it’s all about the function of the microbial community,” said Grandy.
“Our model is arguing that all SOM is derived from the microbes themselves; the bodies of the microbes (necromass) which contain simple compounds, carbohydrates, nitrogen-bearing compounds such as amino acids, proteins, and lipids. These products of microbial biomass are not inherently resistant to decomposition — they just stick to clay minerals and it’s that association between the clay and the residue that protects the microbial fragments and turns them into long term, stable SOM.”
The importance of crop rotation
Plant inputs are still very important to SOM formation. Not because they are contributing directly, as was once thought, but because they influence and shape the microbial communities that ultimately become SOM. Grandy demonstrated this in a field experiment where he compared an organic and conventional rotation.
Grandy found that the microbial growth rate and efficiency in the organic system was higher than in the conventional system, and there was more soil carbon present, even though there was less biomass input and more tillage in the organic system.
Based on conventional wisdom about SOM formation, this seems counterintuitive.
“We typically expect that carbon inputs are associated with soil carbon, so the higher the carbon inputs from crop residue, the higher the soil carbon should be,” says Grandy. “We also expect that increases in tillage reduces SOM especially in sandy type soils like those in the experimental fields, and yet we are seeing an increase in SOM.”
The reason was the quality and timing of the carbon inputs. “A clover cover crop is a really important component of the rotation in the organic system,” says Grandy. “It diversifies the carbon inputs, changes the timing of those inputs, and increases the inputs of carbon that have a low C to N ratio. Basically, the microbial growth rate and efficiency increases with high quality carbon inputs like clover residue. The rotation that included a clover cover crop had a microbial community that more efficiently converted inputs into SOM.”
Grandy’s research opens up opportunities to rejuvenate depleted soils and build stable, long lasting SOM quickly.
“Diversifying our crop rotations and including cover crops is especially important for promoting these microbial communities that are going to facilitate SOM formation,” he says.
Any system can build soil
The potential exists for any farming system, whether it’s organic, conventional or no-till to encourage microbial communities that will build SOM efficiently. “Building healthy soils with adequate SOM depends upon building a diverse and efficient microbial community and the best way to do that is by diversifying the kinds of plant and animal inputs that are going into soils through perennials, inter-cropping, cover crops, or increasing rotation lengths,” says Grandy. “From a SOM perspective the key is promoting more efficient, diverse and larger microbial communities. There is a lot of promise here to develop local and region-specific strategies for cultivating the microbial communities that we need to build SOM.”
Grandy is experimenting with intensive grazing management to see what effect it has on microbial communities. “We are finding that more intensive grazing promotes more root production and root exudates that stimulates microbial biomass and also promotes a more diverse plant community which can translate into a more diverse and efficient below ground community,” says Grandy.
Some cattle producers are already ahead of the curve, discovering they can use plant diversity and changes in their management style to increase SOM, rejuvenate tired soils and increase productivity on the same number of acres.
“When producers change their grazing management, in a few years, the results are incredible,” says Thiele. “The soil biology starts to function again, the nutrient cycle starts to work, and the result of that is higher soil carbon, better nutrient cycling, and better water infiltration.”
A great example of that process can be seen on Brian Harper’s farm. Harper bought a beaten up piece of land beside the Trans-Canada near Brandon that had been in brome and alfalfa for many years. The first thing Harper did was add plant diversity by sod seeding in some legumes such as milkvetch, alfalfa and purple prairie clover. He raised his stock density for grazing to 70,000 lbs. of beef per acre from the 3,000 lbs. per acre that had been grazing on the land previously.
“When the cows graze you don’t let them take too much — just one bite,” says Harper. “The rest they trample down to the ground to provide litter which keeps the ground covered, retains moisture and keeps the soil temperature down. Meanwhile the soil microbes are stimulated and begin to break down the litter and increase SOM.”
Harper saw immediate benefits even in the first year, doubling his carrying capacity. The key to getting more productivity off that piece of land was the grazing management. Harper divided the pasture into long, narrow, half-acre paddocks, moving the cattle every six hours, and not returning to re-graze for at least 80 to 90 days.
“The key to the system is short grazing periods and long recovery periods to allow the grass to re-grow,” says Thiele, who collaborated with Harper on the project, which received some funding from the Commission for Environmental Co-operation, administered through the Manitoba Forage & Grasslands Association.